The present invention relates to parallel plate reactive ion etching, and in particular to increasing the uniformity of the etch rate.
In a parallel plate reactive ion etch (RIE) device, two plates, an upper and a lower electrode plate (anode and cathode respectively) are disposed within a pressure controlled chamber. One or more wafers to be etched are placed on the lower plate and are subjected to an ion bombardment brought about by a high radio frequency (RF) voltage across the plates. An inert gas, such as CF.sub.4 is introduced between the plates as a source of ions, referred to as reactive species. In the RIE process, the upper plate is grounded to the chamber, and an RF voltage source is coupled to the lower plate. This arrangement causes a self-bias of several hundred volts negative to form on the lower electrode, which accelerates the reactive gas species generated from the process gas vertically toward the wafers on the electrode. The reactive gas species thus reacts chemically and physically, as by bombardment, with the wafers. This combination of chemical/physical interaction with the wafer, using the correct process gas chemistry, results in vertical, or anisotropic etching of patterns on the wafer. There is very little sideways etching under a photoresist mask which defines the areas to be etched. Thus, fine line widths may be obtained using an RIE process.
One difficulty in the RIE process is that of obtaining a uniform etch rate of a thin film on a batch of wafers placed on the cathode. Measurements have shown that the rate of etch of the thin film can be substantially greater on wafers situated closer to the edge of the cathode compared with wafers close to the center of the cathode. The nonuniformity is often severe enough to prevent easy implementation of the RIE process in mass semiconductor manufacturing. Attempts have been made to improve uniformity of etch by improving the uniformity of process gas distribution within the RIE chamber based on the theory that local depletion of reactive species in the process gas produces different etch rates at various distances from point at which gas is introduced. Since the process gas was found not to be depleted of reactive species anywhere in the chamber, these efforts did not work. Similarly, changing the plate spacing did not change the uniformity of the etch.
Plasma etching uses a similar arrangement of plates in a chamber, but the RF source is coupled to the top plate, resulting in a different type of etch. The absence of a substantial negative self-bias on the lower plate in a plasma etching process (not using RIE) produces an isotropic reaction with the wafers which is closer to a chemical reaction. Sideways etching under the mask occurs, preventing the use of this process for fine line widths. There is no bombardment, as with an RIE process. Distribution of the species producing gas in a plasma etching process, as opposed to an RIE process, has been an important consideration. In U.S. Pat. No. 4,297,162 to Mundt et al., a top plate is seen with a plurality of holes for distribution of the gas. The holes serve no purpose other than to distribute the gas. In the Mundt et al. patent, the top plate is also curved to create a more uniform etch. There is no need for such a gas distributing mechanism in an RIE apparatus.